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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > NA Engine (non-turbo) / Drivetrain / Exhaust Modifications > 3.91 final drive gearing and revs at speeds...



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      06-11-2018, 10:55 AM   #1
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3.91 final drive gearing and revs at speeds...

In case anyone was wondering with a MT in 6th Gear it’s 3k at 70mph



And it’s about 3900 at 90mph



Very happy with this gearing the car feels so alive and responsive. I came from a prior 3.73 swap (originally 3.23) and notice the little change from 3.73 to 3.91 in a good way.
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      06-11-2018, 12:31 PM   #2
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Quote:
Originally Posted by Biginboca View Post
In case anyone was wondering with a MT in 6th Gear itís 3k at 70mph



And itís about 3900 at 90mph



Very happy with this gearing the car feels so alive and responsive. I came from a prior 3.73 swap (originally 3.23) and notice the little change from 3.73 to 3.91 in a good way.
Thanks for the info! I'm looking at gearing now and this is very helpful. If you don't mind me asking, where did you get the parts to make yours an LSD? I'm looking to do the same as well. What was the approximate cost to convert the whole diff?
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      06-11-2018, 01:06 PM   #3
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Tell me more about your steering wheel.
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      06-11-2018, 01:23 PM   #4
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Tell me more about your steering wheel.


Great wheel very high quality and well made I purchased from eBay. Took about a month to arrive I offered $190 and was accepted with $70 shipping. So $260 on eBay delivered, and I’m pretty sure there’s a guy reselling them on here for like $500.

https://www.ebay.com/itm/BMW-E90-E91...-/112894906677

I added a carbon overlay to cover my peeling sport wheel trim. I also got that off eBay paid about $80 for that. So I’m like $340 into the wheel set up pictured above.
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      06-11-2018, 01:26 PM   #5
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Quote:
Originally Posted by mrnuke View Post
Thanks for the info! I'm looking at gearing now and this is very helpful. If you don't mind me asking, where did you get the parts to make yours an LSD? I'm looking to do the same as well. What was the approximate cost to convert the whole diff?
I bought a used 3.91 diff off eBay (are you guys sensing a theme here lol!?!?) for about $200 delivered.

Sent that diff to M Factory (Synchrotech pm’ed on this forum) and they rebuilt it, swapped the 3 bolt pinion flange to the 4 bolt I needed, and installed a Helical LSD. Great service from them they had it turned around in 48 hours.

Last edited by Biginboca; 06-11-2018 at 01:35 PM.
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      06-11-2018, 01:41 PM   #6
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Quote:
Originally Posted by Biginboca View Post


Great wheel very high quality and well made I purchased from eBay. Took about a month to arrive I offered $190 and was accepted with $70 shipping. So $260 on eBay delivered, and Iím pretty sure thereís a guy reselling them on here for like $500.

https://www.ebay.com/itm/BMW-E90-E91...-/112894906677

I added a carbon overlay to cover my peeling sport wheel trim. I also got that off eBay paid about $80 for that. So Iím like $340 into the wheel set up pictured above.
My Msport one is peeling and starting to look like shit - for $190 - that's killer.
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      06-11-2018, 01:53 PM   #7
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Quote:
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My Msport one is peeling and starting to look like shit - for $190 - that's killer.
If you like the shape of your stock wheel and like DIY projects the Xuji wrap off amazon is also very nice. I had this before and installed myself:



It only costs like $60 and took like 1 hour to stitch on. Need to remove the wheel to stictch properly I missed a few stitches so it wasn’t perfect but looked and felt really great. Can’t fault the material or fit at all just my install errors bugged me. Also the Xuji wrap fattened up the stock sport wheel nicely, better result than a swapped M3 wheel IMO!
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      06-11-2018, 02:22 PM   #8
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Quote:
Originally Posted by Biginboca View Post
In case anyone was wondering with a MT in 6th Gear itís 3k at 70mph


Very happy with this gearing the car feels so alive and responsive. I came from a prior 3.73 swap (originally 3.23) and notice the little change from 3.73 to 3.91 in a good way.
Just fyi...

What you are showing is not a true speed. Your speedo is lying.

If you have any coding capabilities, enable digital speedometer and do "true speed". True speed will match GPS speed. Analog display overstates the speed ("to keep you safe") by about 5-7%. So, you will have to go about 95 to be truly at 90.
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      06-11-2018, 02:34 PM   #9
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Quote:
Originally Posted by W37V View Post
Just fyi...

What you are showing is not a true speed. Your speedo is lying.

If you have any coding capabilities, enable digital speedometer and do "true speed". True speed will match GPS speed. Analog display overstates the speed ("to keep you safe") by about 5-7%. So, you will have to go about 95 to be truly at 90.
Yes I know this is true, I have a few GPS speedometers (Dragy and Phone) but thought it was a 3mph difference or so. I didnít realize it was as high as 7%, but never really checked it. Maybe mines closer cause I have 275/35 rear tires they are a little larger diameter?
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      06-11-2018, 05:36 PM   #10
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Quote:
Originally Posted by Biginboca View Post
Yes I know this is true, I have a few GPS speedometers (Dragy and Phone) but thought it was a 3mph difference or so. I didnít realize it was as high as 7%, but never really checked it. Maybe mines closer cause I have 275/35 rear tires they are a little larger diameter?
That's VERY wide. I'm not sure how I feel about that - pictures?
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      06-11-2018, 05:44 PM   #11
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Quote:
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That's VERY wide. I'm not sure how I feel about that - pictures?
It seems good not too wide. I’m running 255/35 front and 275/35 rears. I tried to take a few pics just now but they didn’t come out great. Will try again when the lighting is better.






Last edited by Biginboca; 06-11-2018 at 06:11 PM.
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      06-11-2018, 06:52 PM   #12
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Is it just me or does the E92 actually look better than the Cayman?
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      06-11-2018, 07:07 PM   #13
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Is it just me or does the E92 actually look better than the Cayman?
I’m with you. The only cayman that looks good to me is the GT4. I happen to think the e92 is one of the best looking cars ever and I seldom see a car I like the looks of more, that’s why I bought one.

It’s interesting how sharp the creases are in the body lines of the e92. The Porsche has a much more organic look with soft corners, I like the razor sharp creases better.






Last edited by Biginboca; 06-11-2018 at 07:25 PM.
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      06-12-2018, 08:46 AM   #14
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Quote:
Originally Posted by Biginboca View Post
It seems good not too wide. Iím running 255/35 front and 275/35 rears. I tried to take a few pics just now but they didnít come out great. Will try again when the lighting is better.





Everything went better than expected.
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      06-12-2018, 02:19 PM   #15
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Thanks for taking that 90mph pic for yours truly.
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      06-12-2018, 04:42 PM   #16
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Thanks for taking that 90mph pic for yours truly.
Yep that was for you lol
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      06-13-2018, 11:38 PM   #17
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Can you record any acceleration runs with the tach/speedo?

I love how aggressive this is
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      06-14-2018, 10:25 AM   #18
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Just curious. How short is 1st gear now? I just did a 3.73 swap from 3.15 and first is laughably short now. I don't really mind it though. Well worth it IMO.
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      06-14-2018, 10:47 AM   #19
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Quote:
Originally Posted by GreaseJunkie View Post
Just curious. How short is 1st gear now? I just did a 3.73 swap from 3.15 and first is laughably short now. I don't really mind it though. Well worth it IMO.
1st tops out just over 30 (32ish on my speedo and 7300rpms) and 2nd just about 59 (maybe 61 on my speedo at about 7500 rpms).

I don’t notice much difference in those gears really, but third gear I’m seeing a bit of a noticeable drop it tops out just under 90 on my speedo and used to be like 94mph.

These are comparing 3.73 and 3.91 (not from stock 3.23)
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      06-14-2018, 10:54 AM   #20
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Can you record any acceleration runs with the tach/speedo?

I love how aggressive this is
Will do soon!
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      06-14-2018, 11:09 AM   #21
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Quote:
Originally Posted by GreaseJunkie View Post
Just curious. How short is 1st gear now? I just did a 3.73 swap from 3.15 and first is laughably short now. I don't really mind it though. Well worth it IMO.
Quote:
Originally Posted by Biginboca View Post
1st tops out just over 30 (32ish on my speedo and 7300rpms) and 2nd just about 59 (maybe 61 on my speedo at about 7500 rpms).

I don’t notice much difference in those gears really, but third gear I’m seeing a bit of a noticeable drop it tops out just under 90 on my speedo and used to be like 94mph.

These are comparing 3.73 and 3.91 (not from stock 3.23)
3.23, 3.46 (N54 auto diff) 3.73 and 3.91 ratios. 25" tire diameter is typical for an 18" wheel.
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Last edited by desertman123; 06-14-2018 at 11:21 AM.
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      06-14-2018, 12:12 PM   #22
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desertman123 thanks for posting that. That seems about right on.

My redline is 7500 so Iím seeing a bit more speed in each gear than those figures also my rear tire specs at 25.6Ē diameter so I get a little bump on speed in each gear over what that shows.
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